Processes for producing alpha olefins from ethylene generally lead to an ensemble of oligomers containing 4 to 30 carbon atoms, or even more than 30, and the olefins are then separated by distillation. Over the past few years, demand for lower oligomers, essentially 1-butene, 1-hexene and 1-octene, which are used as co-monomers with ethylene for the manufacture of linear low density polyethylene, has been increasing.
Few catalysts exist which selectively produce a particular oligomer, as is the case when dimerizing ethylene to 1-butene with a titanium based catalyst. However, chromium based catalysts are known to lead to the formation of mainly 1-hexene, with polyethylene to a greater or lesser extent, the proportion of butenes and octenes in the products being very small (R. M. Manyik, W. E. Walker, T. P. Wilson, J. Catal., 1977, 47, 197 and J. R. Briggs, J. Chem. Soc., Chem. Commun. 1989, 674 and references cited therein). Catalysts which enable selective trimerization of ethylene have recently been claimed (U.S. Pat. No. 5,198,563, U.S. Pat. No. 5,288,823, U.S. Pat. No. 5,382,738, European patent EP-A-0 608 447, EP-A-0 611 743, EP-A-0 614 865). Such catalysts are prepared from a chromium salt and a metal amide, in particular a pyrrolide. Other catalysts use an aluminoxane and a chromium complex with a chelating phosphine (U.S. Pat. No. 5,550,305).